1. Field of the Invention
The present invention relates to a method of manufacturing a smooth foamed polypropylene resin sheet having a large width of 1 m or more.
2. Description of the Related Art
Foamed polypropylene resin sheets having closed cells have been currently used in a variety of applications such as automobile interior core materials as well as boxes and storage cases as reformed products since these sheets have high rigidity.
Several methods are proposed for extrusion-molding of foamed thermoplastic resin sheet. Typical of these conventional methods are a tenter method in which a foamed sheet extruded by an extruder is taken up while being stretched widthwise and a method using a molding die with a Teflon-coated inner surface which is located near a T-die to control the outer diameter.
According to the former method, a waved sheet extruded and foamed from the T-die is expanded in the transverse direction by a tenter. It is difficult to form a sheet having a uniform thickness if the width of the sheet exceeds 1 m or more. In addition, the central portion of the sheet undesirably becomes thicker than the peripheral portion, and vertical stripes tend to be formed.
According to the latter method, it is difficult to match molding speed with that of a take-pp apparatus. If the thickness of the product is as small as 3 mm or less, it is difficult to allow proper molding because a frictional force between the inner surface of the molding die and the extruded molten resin is large.
Ween a foamed polypropylene resin sheet is manufactured according to the above typical conventional extrusion-molding for forming a foamed thermoplastic resin sheet, in addition to the above problems, the surface of the resultant foamed sheet is roughened at an expansion ratio of 2 or more. As a result, the outer appearance of the foamed sheet is worse.
Still another conventional method of manufacturing a foamed polystyrene sheet or the like is proposed in Japanese Patent Publication No. 33-4584 wherein two molding rolls are disposed adjacent to a T-die lip to form a surface of the extruded foamed sheet. This method allows easy extrusion-molding of a wide sheet. In addition, sheets can be easily formed in a wide range of thickness.
In order to develop a manufacturing method of a foamed polypropylene resin sheet having a smooth surface and an expansion ratio of 2 or more, the present inventors made extensive studies based on the method using two molding rolls disposed adjacent to the T-die lip and found that when ethylene propylene rubber was mixed in a propylene-ethylene block copolymer to prepare a composition and the composition was foamed into a sheet in such a manner that a T-die was used and small-diameter rolls were set close to the T-die to cool the two surfaces of the sheet immediately after extrusion while a distance between the rolls and an apex of a die was set at (R.sup.2 +16).sup.1/2 or less (R: radius of roll), an excellent foamed sheet was obtained (Japanese Patent Application No. 57-226930; Japanese Patent Disclosure (Kokai) No. 59-120430). In this method, however, the following problem arose when the material was processed over a long period of time while being maintained stable, into a foamed sheet having a width of 1 m or more.
In order to obtain a sheet having a width of 1 m or more, an extruder of 80 mm or more had to be used to perform extrusion while a head pressure of at least 80 kg/cm.sup.2 was applied to the foamable composition. When the diameter of the roll die was 30 mm or less, the roll die tended to be deformed by the resin pressure and manufacturing of the roll itself was very difficult.
In view of this, the diameter of the roll was set at 30 mm or more, and the rolls were set closer to the T-die so as to bring the extrudate into contact with the rolls before foaming. Then, the rolls were brought into partial contact with the die, and the die was cooled. As a result, a stable foaming state could not be obtained for a long.